Method and apparatus for the determination of the relative frequency offset between an input optical signal and a resonance frequency of an optical cavity

Abstract

A method and apparatus for stabilizing the relative frequency offset between a laser and a resonant optical cavity is described. Detected spatial mode patterns excited in a coupled nearly confocal resonator are used to derive a correction signal for frequency offset. The method can be applied to stabilizing a diode-cavity pathlength for an optical feedback locked diode laser system.

What we claim is: 
     
       1. A method of providing a control signal comprising the steps of: a) providing a resonant cavity having a plurality of longitudinal and transverse modes inherently associated with resonance of the cavity;   b) providing the resonant cavity with an optical signal having a spatial intensity mode pattern that is suitable for excitation of at least one of the longtitudinal modes and a plurality of associated transverse modes of the cavity;   c) allowing a plurality of the transverse modes to become excited within the resonant cavity in the presence of the optical signal, each transverse mode having a resonant frequency;   d) detecting relative intensities of a selected plurality of transverse modes that are excited within the cavity; and   e) generating a control signal by comparing the relative intensities determined in step (c), the control signal being representative of the relative frequency difference between the optical signal and the resonance of the resonant cavity.   
     
     
       2. A method as defined in claim 1, wherein the step of determining the relative intensities of the plurality of the transverse modes includes the step of detecting at a plurality of locations adjacent the cavity, the power of the plurality of transverse modes that are excited within the cavity. 
     
     
       3. A method as defined in claim 2, wherein the step of detecting is performed by a plurality of optical detectors. 
     
     
       4. A method as defined in claim 1, comprising the step of varying one of the frequency of the optical signal and the resonant frequency of the resonant cavity in dependence upon the intensity of the control signal. 
     
     
       5. A method as defined in claim 1, further comprising the step of relatively locking the resonant frequency of the resonant cavity to the frequency of the optical signal by providing the control signal as a feedback signal to the resonant cavity. 
     
     
       6. An apparatus for providing a control signal characteristic of a relative frequency of an optical signal with a resonance of a resonant cavity having a plurality of associated transverse modes, comprising: laser means for generating an optical signal, the optical signal having a spatial intensity mode pattern for excitation of the plurality of associated transverse modes of the resonant cavity;   the resonant cavity having an input port for receiving the optical signal from the laser means, and having an output port for accepting the optical signal after it has been excited within the resonant cavity; means for detecting at a plurality of locations the intensities of a plurality of transverse modes excited within the cavity after propagating through the output port of the resonant cavity; and, means for transforming the detected intensities into a control signal for controlling a characteristic of one of the laser means and the resonant cavity.   
     
     
       7. An apparatus as defined in claim 6, wherein the laser means and the resonant cavity are a part of a same optical resonator for the laser source in which the derived control signal is used to maintain resonance. 
     
     
       8. An apparatus as defined in claim 6 including a non-linear optical medium within the resonant cavity for enhancing the generation of harmonic radiation.